The long range objectives of this research are: (1) to achieve an understanding of optical phenomena in complex molecular systems, including electronic absorption and circular dichroic (CD) spectra and related effects arising from molecular polarizabilities; and (2) to develop an improved basis for interpreting such optical phenomena in terms of molecular structures. The specific aims are: (1) to continue studies of peptide absorption and CD spectra using the theory developed under this project; (2) to apply the multiple interaction theory developed in this project to the calculation of absorption and CD spectra; (3) to apply similar models to molecules containing aromatic chromophores; (4) to examine the effects of solvent on absorption spectra by the theory of a cavity model for the solvent; and (5) to extend the theory to related phenomena, including magnetic effects and nonlinear optical effects in electronic absorption. The importance of this project to the health sciences lies in its bearing on phenomena that are widely used to study structure and behavior of complex biomolecules.